hi there! sorry to contradict selina but it's the other way round: ARE YOU SERIOUS? = correct form/ question You need an auxiliary (be) to form the question.In this case BE turns into ARE because of the subject YOU. YOU SERIOUS* = this form is incorrect and doesn't exist in proper English. Here Selina is right. Something is left out... You can call it OMISSION or ELLIPSIS or ELLIPTICAL SENTENCE. If you have problems in forming questions try this site to get an overview: http://www.ecenglish.com/learnenglish/lessons/ask-it-right-practise-asking-questions and check your grammar book..there must be either a chapter about 'auxiliary verbs' or 'how to form questions'! Good luck, patty!!

Both are correct. 'You serious" is slang. or the shorter version.

they're the same

the first one is an ellipsis sentence, it is usually used in a formal occasion, while the second one is obviously correct.